Euler's polyhedron formula asserts for a polyhedron p that V - E + F = 2, where V , E, and F are, respectively, the numbers of vertices, edges, and faces of p. This paper concerns a formal proof in the mizar system of Euler's polyhedron formula carried out  by the author. We discuss the informal proof (Poincaré's) on which the formal proof is based, the formalism in which the proof was carried out, notable features of the formalization, and related projects.
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